Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 147-151
lead to operational problems in preheaters or short residence time reactors. At long reaction times (> 3 min), chemical changes occur less rapidly. Kinetics developed by Cronauer et al. (1978) can be used to describse these long time reactions with this coal and these product definitions. Further study is needed to define changes occurring at early reaction times. L i t e r a t u r e Cited Bickel, T. C., unpublished data, Sandia Laboratories, Oct 1978. Cronauer, R. G., Roberto, R. G., Shah, Y. T., Proceedings of the EPRI Contractors' Conference on Coal Liquefaction, Electric Power Research Institute/Palo Alto, CA, p 4-1, May 1978. Curlee, R. M., Hawn, D. N., SAND78-0837, Sandia LaboratorieslAlbuquerque, NM, July 1978. Kleinpeter, J. A., Burke, F. P., Proceedings of EPRI Contractors' Conference on Coal Liquefaction, ElecWic Power Research InstiRutelPalo Alto, Ca, p 11-1 May 1978. Lewis, H. E., Weber, W. H.. Usnick, G. B., Hollenack, W. R., Bhir, H. O., Baykin, R. G., Quarterly Technical Progress Report for July-Sept 1978, Catalytic Inc.lWilsonville, Ala. (in process). Neavel, R. C., Fuel, 55(3), 237 (1976). Plett. E. G., Alkidas, A. C., Flogers, F. E., Summerfield, M., Fuel, 58(3), 241 (1977).
147
Scheller, J. E., Farnum, B. W., Sondreal, E. A,, Am. Chem. SOC. Div. fuel Chem. Prepr., 22(6) 33 (1977). Thomas, M. G., private communication, Nov 1978. Thomas, M. G.. Granoff, B., Fuel, 57(2), 122 (1978). Thomas, M. G., Noles, G. T., SAND78-0088, Sandia Laboratwies/Albuquerque, NM, April 1978. Traeger, R . K., Bickel, T. C., Curlee, R . M., Annual Report for October 1977 to September 1978, SAND79-0150, Sandi Laboratories/Aibuquerque,NM. May 1979. Traeaer, R. K.. Curlee, R. M., SAND78-1124, Sandi LaboratorieslAlbuquerque, NM, June 1978. Traeger, R. K.,Curlee, R. M., SAND78-1872, S a d i LaboratorieslAlbuquerque, NM . ...., oct - -. 1978 . .. - . Whitehurst, D. D., Proceedings of EPRI Contractors' Conference on Coal Liquefaction, Electric Power Research InstitutdPalo Alto, Ca, p 1-1, May 1978.
Received for review May 4, 1979 Accepted January 7, 1980 This work was supported by the United States Department of Energy (DOE) under Conract Number DE-AC04-76DP00789, This paper was presented at the American Institute of Chemical Engineers National Meeting in Houston, TX, Apr 2, 1978.
IV. 53rd Colloid and Surface Science Symposium Rolla, Missouri, June 1979
Formation of Ultrafine Fe,O, Aerosols from a Flame Supported Reaction Pierre! G. Vergnon" and Habib Batis Landoulsi Laboraltoire de Catalyse AppliquBe et CinBtique HBtBrogBne de I'UniversitB Claude Bernard (Lyon I) associ6 au C.N.R.S. (L.A. No. 23 1). 139622 Villeurbanne Cedex, France
Ferric oxide is prepared in an oxhydric flame from FeCI, vapor. Gas phase reaction, nonequilibrium conditions, and the temperature distribution in the flame allow one to obtain nonporous oxide particles under different sizes (10 to 200 nm) and crystalline forms (yor a).A formation mechanism of oxide particles in the flame is proposed. A variation of the lattice parameters is observed in the case of smallest particles. The relative intensity of superstructure lines observed in y-ferric oxide depends on the particle size and it can be correlated to the magnetic properties characterized by the magnetization at infinite field.
Introduction Ultrafine powders (size To,> To,),the more elevated is condensed phase (TO3 the nucleation temperature To> Tz > T , (see Figure 2A). When the supersaturation increases, the nucleation temperature increases (Figure 2B) and the critical nucleus radius decreases (eq 3, Figure 2C). The nature of the condensed phase has not yet been determined. If the vapor is formed at a temperature below the melting point of the solid phase, the nuclei appear directly in the solid state. This event occurs when the nucleation temperature is below a temperature To and therefore for a small supersaturation (or for a small partial pressure of the supersaturated phase, P < P,, see Figure 2D). Growth of Condensed Phase. Following the nucleation the growth process of stable nuclei takes place by
Ind. Eng. Chem. Prod. Res. DQV., Val. 19. No. 2. 1980 149
B
01
, 0
100
, 200
p p b l
, 300
press-
,
,
400 500 o i Fe2C16
, 6OC
, I *
700 IPoI
Figure 3. Influence of the partial pressure of iron chloride vapor on the size of particles ( 0 ) and crystallites (0)of a-FelOs. Figure 2. A, Variation of the nucleation rate with the equilibrium temperature of the supersaturated vapor; B, variation of the nucleation temperature with the supersaturation; C, variation of the critical nucleus radius with the supersaturation; D,phase diagram of nuclei.
different mechanisms: (i) diffusion of the condensing species to the particle surface and condensation on the surface. In the case of Fe,03 reaction a t the phase boundary must he involved since vapor molecules are not of the same composition as the particles; (ii) collision promoted by Brownian motion and coalescence of particles. When crystalline solid nuclei are directly formed from the vapor phase, their growth by the diffusion-condensation mechanism leads to polyhedral particles, whereas liquid droplets take a spherical form. In hoth cases their size increases with the partial pressure of chloride (Ghoshtagore, 1970). Simultaneously, if the concentration of solid or liquid particles in the flame is important, collision can occur. The characteristics of collisions vary widely depending on whether the particles stick and retain their individual identities (giving polycrystalline particles) or, a t the other extreme, fuse completely to form a new single particle. The sintering rate of ultrafine oxide particle can be large enough to be fully operative. The shapes are characteristic since sintering involves the formation of smooth surfaces. Solidification of Liquid Droplets. If liquid droplets are formed, they must undergo a liquid-solid transformation. Out of the flame the temperature is rapidly decreasing and liquid droplets are quenched. The solid particles can maintain the spherical shape (with or without facets), and moreover metastable crystalline structures can be obtained. This liquidsolid transformation determines the structural organization of oxide particles, and the procedure of cooling the particles is an important factor for the control of the final properties of particles. Unfortunately, this effect depends on the particle size, and its independent variation is not easy to control. Experimental Results The conditions of aerosol formation in the flame reactor described are innumerable. Moreover this reactor is particularly convenient for the preparation of doped oxides or solid solutions. Thus, only two typical ways of using of the burner (cold and hot flames) are described concerning the formation of pure Fe203. Cold Flame (1200 K). All particles prepared in a cold flame are polyhedral more or less sintered. They have the rhombohedral structure of a-Fez03. Particle diameter calculated from the BET surface area increases with the partial pressure of iron chloride (Figure 3). The smallest particles are of uniform size (Figure 4a), whereas aggregates or sintered particles appear when particle diameter in-
C Figure 4. A, Electron micrographs of a-Fe,O, aerosols (average diameter D = 13 nmj; B, electron micrographs of a-Fe202aerosols (average diameter D = 47 nmj; C, electron micrographs of r-Fe,02 aerosols (average diameter D = 80 nm).
creases (Figure 4b). For each sample the crystallite size estimated from the broadening of the [lo41 and (1101 lines (indexed in the hexagonal system) has been compared with the particle diameter (from BET measurement) (Figure 3). The agreement is good for small particles (
